U.S. patent application number 15/883882 was filed with the patent office on 2018-06-21 for high strength weld metal for demanding structural applications.
The applicant listed for this patent is Raghavan Ayer, Douglas P. Fairchild, Hyun-Woo Jin, Mario L. Macia, Nathan E. Nissley, Adnan Ozekcin. Invention is credited to Raghavan Ayer, Douglas P. Fairchild, Hyun-Woo Jin, Mario L. Macia, Nathan E. Nissley, Adnan Ozekcin.
Application Number | 20180169799 15/883882 |
Document ID | / |
Family ID | 49997721 |
Filed Date | 2018-06-21 |
United States Patent
Application |
20180169799 |
Kind Code |
A1 |
Fairchild; Douglas P. ; et
al. |
June 21, 2018 |
High Strength Weld Metal for Demanding Structural Applications
Abstract
Weld metals and methods for welding ferritic steels are
provided. The weld metals have high strength and high ductile
tearing resistance and are suitable for use in strain based
pipelines. The weld metals are comprised of between 0.03 and 0.08
wt % carbon, between 2.0 and 3.5 wt % nickel, not greater than
about 2.0 wt % manganese, not greater than about 0.80 wt %
molybdenum, not greater than about 0.70 wt % silicon, not greater
than about 0.03 wt % aluminum, not greater than 0.02 wt % titanium,
not greater than 0.04 wt % zirconium, between 100 and 225 ppm
oxygen, not greater than about 100 ppm nitrogen, not greater than
about 100 ppm sulfur, not greater than about 100 ppm phosphorus,
and the balance essentially iron. The weld metals are applied using
a power source with pulsed current waveform control with <5%
CO.sub.2 and <2% oxygen in the shielding gas.
Inventors: |
Fairchild; Douglas P.;
(Sugar Land, TX) ; Macia; Mario L.; (Bellaire,
TX) ; Nissley; Nathan E.; (Houston, TX) ;
Ayer; Raghavan; (Daejeon, KR) ; Jin; Hyun-Woo;
(Easton, PA) ; Ozekcin; Adnan; (Bethlehem,
PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fairchild; Douglas P.
Macia; Mario L.
Nissley; Nathan E.
Ayer; Raghavan
Jin; Hyun-Woo
Ozekcin; Adnan |
Sugar Land
Bellaire
Houston
Daejeon
Easton
Bethlehem |
TX
TX
TX
PA
PA |
US
US
US
KR
US
US |
|
|
Family ID: |
49997721 |
Appl. No.: |
15/883882 |
Filed: |
January 30, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14408239 |
Dec 15, 2014 |
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PCT/US13/47384 |
Jun 24, 2013 |
|
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15883882 |
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61676738 |
Jul 27, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/50 20130101;
B23K 9/186 20130101; C22C 38/42 20130101; C22C 38/58 20130101; B23K
35/3086 20130101; B23K 35/383 20130101; C22C 38/12 20130101; C22C
38/44 20130101; C22C 38/002 20130101; C22C 38/16 20130101; B23K
35/308 20130101; C22C 38/04 20130101; C22C 38/00 20130101; C22C
38/14 20130101; C22C 38/08 20130101; B23K 9/173 20130101; C22C
38/02 20130101; B23K 35/3066 20130101 |
International
Class: |
B23K 35/30 20060101
B23K035/30; C22C 38/58 20060101 C22C038/58; B23K 9/173 20060101
B23K009/173; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/16 20060101 C22C038/16; C22C 38/14 20060101
C22C038/14; C22C 38/12 20060101 C22C038/12; C22C 38/08 20060101
C22C038/08; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; B23K 35/38 20060101
B23K035/38; B23K 9/18 20060101 B23K009/18; C22C 38/50 20060101
C22C038/50 |
Claims
1.-22. (canceled)
23. A method of welding ferritic steel pipelines comprising:
determining a desired HSW weld metal chemistry comprising between
0.03 and 0.08 wt % carbon, between 2.0 and 3.5 wt % nickel, not
greater than about 2.0 wt % manganese, not greater than about 0.80
wt % molybdenum, not greater than about 0.70 wt % silicon, not
greater than about 0.03 wt % aluminum, not greater than 0.02 wt %
titanium, not greater than 0.04 wt % zirconium, between 100 and 225
ppm oxygen, not greater than about 100 ppm nitrogen, not greater
than about 100 ppm sulfur, not greater than about 100 ppm
phosphorus, and the balance iron; determining and providing a
welding consumable wire chemistry from a calculation using as
inputs dilution percent, a pipeline base metal chemistry, and the
desired HSW weld metal chemistry; and girth welding the pipeline
base metal using the welding consumable wire to produce a weld
metal, the girth welding process comprising: applying the girth
welding using a gas metal arc welding process using a shielding gas
with less than 5 vol % CO.sub.2 and less than 2 vol % O.sub.2, and
using an advanced pulsed waveform power supply constructed and
controlled to mitigate the negative weldability aspects of using a
shielding gas with less than 5 vol % CO.sub.2, wherein the weld
metal achieves a target weld metal oxygen content that is not
greater than about 225 ppm oxygen and a weld metal inclusion
population not greater than 4.times.10.sup.10 m.sup.-2, the weld
has an SBD-AFIM microstructure, a tensile strength of greater than
90 ksi and a SENT R-curve delta value of greater than 0.75.
24. The method of claim 23, wherein the shielding gas comprises a
mixture of less than 5 vol % CO.sub.2, helium, and argon in the
amount of at least 50 volume percent.
25. The method of claim 23, wherein the shielding gas comprises a
mixture of less than 5 vol % CO.sub.2, at least 10 vol % helium,
and argon in the amount of at least 50 volume percent.
26. The welding method of claim 23, wherein the shielding gas
comprises a mixture of less than 5 vol % CO.sub.2 and the balance
being argon.
27. The method of claim 23, wherein the step of girth welding
further comprises using a hybrid laser arc welding process.
28. The method of claim 23, wherein the step of girth welding
further comprises using a submerged arc welding process.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority benefit of U.S.
Provisional Patent Application 61/676,738 filed 27 Jul. 2012
entitled HIGH STRENGTH STEEL WELD METAL FOR DEMANDING STRUCTURAL
APPLICATIONS, the entirety of which is incorporated by reference
herein.
FIELD OF THE INVENTION
[0002] This invention relates to the field of welding metals. More
particularly, the invention relates to materials and methods for
producing weld metal having high strength and high toughness.
BACKGROUND
[0003] This section introduces various aspects of the art, which
may be associated with exemplary embodiments of the present
invention. This discussion will assist in providing a framework to
facilitate a better understanding of particular aspects of the
present invention. This section should be read in this light, and
not necessarily as admissions of prior art. In the following
specification, the invention is described in the context of
strain-based design of pipelines. However, the invention is clearly
of wider application to any situation in which a high strength,
high toughness weldment is desirable, including but not limited to
any non-pipe weldments of any one or more steel materials. Various
terms are defined in the following specification. For convenience,
a Glossary of terms is provided immediately preceding the
claims.
[0004] With respect to applied loads, design standards, and
material performance requirements, traditional pipelines are
designed to prevent the pipeline materials from experiencing
significant plastic strains. This type of design is referred to as
allowable stress design or stress-based design. In stress-based
designs, the applied loads are typically limited to some fraction
of the yield strength of the pipe material and the primary design
consideration is pressure containment. In some instances, local
plasticity might occur in a stress-based-designed pipeline at small
stress concentrations like weld toes (i.e., over dimensions of
several millimeters), or at the outer fibers of a bend during pipe
laying, but generally stress-based designs are not intended for
situations where large areas (many inches or feet) of the pipeline
are subjected to plastic strains while the pipeline is
operating.
[0005] Today, pipelines are being designed for increasingly hostile
service environments. The goal of pressure containment design is
still applicable and relevant to circumferential pipe strength, but
some pipelines will also experience service loads in the
longitudinal direction. For some demanding environments such as
discontinuous permafrost, seismic, iceberg scouring, etc. where
service temperatures can range as low as -20.degree. C. or lower,
there is a need to design and build pipelines capable of
withstanding some degree of longitudinal plastic deformation. In
such cases, the deformation is largely oriented parallel to the
pipe axis (i.e., longitudinal plastic strains) and the applied
loads are often described in terms of applied global strains which
are experienced over many inches or possibly feet of pipeline
material. Strain-based design (SBD) is the term used to describe
designing/constructing a pipeline that is capable of incurring
longitudinal plastic strains. Typical strain magnitudes for
strain-based designs are generally defined as global plastic
strains in excess of 0.5%. Global plastic strains are defined as
strains that are not local, but are spread over a distance of many
inches or feel as measured along a length of pipe that may include
one or more girth welds. In the case of an oil or gas pipeline, for
example, global plastic strains for strain-based design purposes
could be in reference to a section of the pipeline that is about
two pipe diameters in length, although other similar definitions
could be used to define global plastic strains. Using this
convention, a global plastic strain of one percent in a 30 inch
diameter pipeline would produce about 0.6 inches of strain in two
diameters of length; i.e., 60 inches in length.
[0006] Fracture mechanics techniques called engineering critical
assessment (ECA) are used to judge the structural significance of
defects in girth welds for stress-based design pipelines. ECA
includes accepted practices for testing materials, qualifying
welds, and assessing the significance of weld imperfections in
stress-based designs. ECA, as applied to stress-based pipelines, is
primarily for the purpose of assessing the significance of girth
weld defects. In such cases, the girth weld defects may see limited
in-service loading in the longitudinal direction and often the most
extreme loading occurs during pipeline installation. This typical
scenario changes with strain-based design (SBD) because of the more
extreme longitudinal in-service loading. Strain-based design is not
as mature a field as traditional stress-based design, and as of
2012, fully validated ECA practices for SBD have not been widely
accepted by the pipeline industry. However, ECA principles are
applicable to SBD. Many aspects of SBD pipeline engineering have
been published at recent international conferences. Several notable
venues include the Conference of Pipeline Technology in Belgium,
the International Pipeline Conference in Canada, and the annual
conferences of The International Society of Offshore and Polar
Engineers (ISOPE) and The Offshore Mechanics and Arctic Engineering
Society (OMAE). ExxonMobil has published numerous articles at these
conferences including topics such as prediction methods for girth
weld defect tolerance under SBD loading conditions, full-scale pipe
testing for SBD engineering, fracture mechanics test methods, and
girth welding technology useful in SBD applications. These
publications in combination with patent applications International
Application Numbers PCT/US2008/001753 (WIPO Patent Application
WO/2008/115323, A Framework To Determine The Capacity Of A
Structure) and PCT/US2008/001676, (WIPO Patent Application
WO/2008/115320, Method To Measure Tearing Resistance) provide the
background necessary for strain-based design engineering critical
assessment (SBECA) technology to one skilled in the art.
[0007] Depending on the service temperature and applied loads,
common structural steels and welds can experience either brittle or
ductile fracture. Ductile fracture occurs at higher temperatures
and brittle (or "cleavage") fracture occurs at lower temperatures.
At some intermediate temperature range, a transition occurs between
ductile and brittle fracture. This transition is sometimes
characterized by a single temperature called the ductile-to-brittle
transition temperature (DBTT). The DBTT can be determined by tests
such as the Charpy V-notch or CTOD test, depending on the
application.
[0008] In stress-based design applications materials engineering
and pipeline design practices are focused on ensuring adequate
brittle fracture resistance and little attention is paid to ductile
fracture of the girth welds. Brittle fracture is mitigated by
specifying a minimum design temperature (consistent with the lowest
anticipated service temperature) and using test methods like the
Charpy V-notch or crack tip opening displacement (CTOD) test to
qualify materials.
[0009] In the newer application of SBD pipelines, however, it is
necessary to go beyond the simple consideration of brittle
fracture; ductile fracture of the girth welds must also be
considered. Girth welds are usually considered potentially the
weakest link due to the common presence of degraded microstructures
and imperfections caused by welding. In SBD, the designer, through
choice of materials, welding, and inspection technology, will
mitigate brittle fracture, or at least delay it until well into the
plastic loading regime and beyond the designed strain demand.
During plastic loading of a pipeline, ductile tearing can initiate
at girth weld discontinuities or defects. Depending on such factors
as the strength properties and ductile tearing resistance of the
welds, discontinuity or defect size, and pipeline base steel, the
amount of tearing can be minimal and stable. If stable, the amount
of defect growth typically ranges from a few microns up to a
millimeter or two. If this degree of growth can be reliably
accounted for in strain-based pipeline engineering practices, and
specifically SBECA procedures, then pipeline integrity can be
quantified and managed. For these reasons, overmatched girth welds
with good ductile tearing resistance are important for SBD
pipelines. There is need for weld metals with high strength and
high tearing resistance. Special testing techniques are recently
available to quantify the tearing properties.
[0010] Naturally, there is an inherent tradeoff between strength
and toughness in structural steels and weldments. As strength
increases, toughness generally decreases. SBD requires both higher
strength and higher toughness. A primary challenge for SBD
pipelines is how to obtain both high strength and high toughness,
particularly tearing resistance, in the girth welds. The properties
of pipeline girth welds are primarily controlled by the
microstructure, which is in turn controlled by the chemistry and
thermal cycle imposed during welding. Chemistry is mostly
controlled by the selection of the welding consumables (wire,
shielding gas, and/or fluxes) and the chemistry of the base
material of the pipe. The weld thermal cycle is primarily a product
of the weld procedure and base material thickness.
[0011] In the pursuit of high strength, high toughness welds,
attempted optimization of properties can result in poor
weldability. When conventional welding techniques are combined with
new metallurgy the result can be poor weld pool fluidity, arc
stability, bead geometry, and penetration profile, all of which can
result in weld defects. This is particularly problematic for
mechanized 5G pipeline girth welds where the constantly changing
weld position and tight bevels creates a challenging situation that
demands a welding method that produces good wetting and stable
consistent operation. Some consumables cannot be welded out of
position for this reason.
[0012] One approach to producing steel pipe welds that are useful
for strain-based design is disclosed in U.S. Patent Application
Publication No. US PA 2010/0089463, published Apr. 15, 2010
(International Patent Application PCT/US2008/001409) which
discloses the use of austenitic filler wires to weld pipe for
strain-based pipeline designs. The publication teaches the
production of high toughness welds using Ni-based alloy, stainless
steel, or duplex stainless steel welding consumables. This weld is
hereafter called the "austenitic SBD weld". This publication
teaches away from ferritic weld metals in that it states
conventional ferritic welds have limitations in toughness and
tearing resistance that restrict the amount of strain that can be
accommodated in structural design. The below application discloses
a ferritic weld that achieves toughness suitable for SBD
applications, but is significantly stronger than the austenitic SBD
weld.
[0013] When austenitic welds are applied to ferritic steels, a
dissimilar atomic structure weld interface is created at the
boundary between the weld metal and the weld heat affected zone
(HAZ). Austenite possesses a face centered cubic (fcc) structure
and ferrite possesses a body centered cubic (bcc) structure.
Application of ultrasonic testing/inspection to dissimilar
interfaces for defects such as lack of fusion can be difficult
because this interface produces sound reflections that can be
misinterpreted. Fcc and bcc materials have different sound
propagation properties and respond differently to ultrasonic
inspection. For challenging applications like SBD, it is desired to
inspect for small defects with a tolerance on the sizing accuracy
on the order of a millimeter. Dissimilar weld interfaces can cause
signals during UT inspection that rival the signals created by
small defects or at least create uncertainties in sizing accuracy.
This is particularly the case for signals that emerge from a
dissimilar weld in an area of the heat affected zone that has other
geometric complexities like cusps or scallops between adjacent weld
beads or in areas where the weld bevel geometry has changed. For
the above reasons, it is desirable that ferritic steel pipelines be
joined with ferritic welds to avoid dissimilar weld interface and
enable accurate inspection when using UT inspection.
[0014] U.S. Pat. No. 6,565,678 (the '678 Patent) discloses a
ferritic weld metal called acicular ferrite interspersed in
martensite (AFIM) that is useful for welding high strength
pipelines. The intended application for this weld metal was not SBD
pipelines. The '678 Patent provides no consideration of SBD
pipelines and the specific requirements of the application herein.
As such, the welds of this prior art have no consideration, design,
or demonstration of achieving high tearing resistance as is needed
for SBD pipelines. No quantification of tearing resistance was made
by the '678 Patent as would be needed for SBECA as described by
literature sources noted above. Furthermore, because the '678
Patent makes no consideration of tearing resistance, it therefore
makes no consideration of the welding techniques required to
produce welds optimum for SBD. This includes use of special
shielding gas mixtures and the resultant need for highly
specialized pulsed waveform power supplies that have only become
available after the invention date of the '678 Patent.
[0015] The utility of oxygen and acicular ferrite in weld metal are
discussed by the '678 Patent; however, there is no attention paid
to optimizing these components for SBD welds. The '678 Patent
states, "For a particular application, the welding engineer can
control the acicular ferrite content and the oxygen level by
choices of weld metal chemistry, shielding gas composition, and
welding procedure (weld cooling rates) according to the guidelines
of this invention". There is no mention of the use of advanced
pulsed waveform power supplies to optimize an AFIM microstructure
for SBD. The consideration of such power supplies would naturally
fall in the category of welding procedure, but the lack of
consideration of such by '678 Patent is understandable because
advanced waveform power supplies were not available at the '678
Patent was filed.
[0016] The '678 Patent discusses the importance of weld metal
inclusions in high strength weld metal. The '678 Patent seeks to
produce a large number of small inclusions in order to nucleate
acicular ferrite. This objective is suitable for conventional high
strength stress based pipelines design where ductile tearing
resistance isn't a primary concern, but the same approach would not
be suitable for SBD pipeline welds. Due to the need for high
tearing resistance, SBD pipeline welds require a lower number of
weld metal inclusions as compared to stress based design welds and
this has been discussed in D. P. Fairchild, et al, "Girth Welds for
Strain-Based Design Pipelines", Proceeding of the 18.sup.th
International ISOPE Conference, Vancouver, 2008.
[0017] There is a need for weld metal that simultaneously produces
high strength, high ductile tearing resistance, and good brittle
fracture resistance (i.e., good ductile and brittle fracture
toughness) and that can be applied during pipeline field
construction without undue concern regarding weldability or ease of
use in terms of weld pool control and defect rates.
SUMMARY
[0018] The present invention provides a novel weld metal that
achieves high strength welds with superior ductile tearing
resistance and good weldability.
[0019] One embodiment of the present disclosure is a weld metal
which comprises between 0.03 and 0.08 wt % carbon, between 2.0 and
3.5 wt % nickel, not greater than about 2.00 wt % manganese, not
greater than about 0.8 wt % molybdenum, not greater than about 0.70
wt % silicon, not greater than about 0.03 wt % aluminum, not
greater than about 0.02 wt % Ti, not greater than about 0.04 wt %
Zr, between 100 and 225 ppm oxygen, not greater than about 100 ppm
sulfur, not greater than about 100 ppm phosphorus, not greater than
about 100 ppm nitrogen, and the balance essentially iron, wherein
the weld metal comprises an SBD-AFIM microstructure, the weld metal
is applied using a pulsed gas metal arc welding process with an
advanced pulsed waveform power supply and utilizes a shielding gas
comprised of less than 5% CO.sub.2 and less than 2% O.sub.2, the
applied weld metal has a tensile strength of greater than 90 ksi
and a SENT R-curve delta value of greater than 0.75.
[0020] In other embodiments of the present disclosure, elements
that may be added to enhance weld metal properties comprise: not
greater than about 0.6 wt % copper, not greater than about 0.04 wt
% vanadium, not greater than about 0.60 wt % Cr, not greater than
about 0.04 wt % Nb, not greater than about 20 ppm B. The carbon
content and other alloys of the weld metal may be adjusted within
the range to provide welds with sufficient strength for SBD
applications with pipe grades X52 to X100 or higher.
[0021] The foregoing has broadly outlined the features of some
embodiments of the present disclosure in order that the detailed
description that follows may be better understood. Additional
features and embodiments will also be described herein.
DESCRIPTION OF THE DRAWINGS
[0022] The present invention and its advantages will be better
understood by referring to the following detailed description and
the attached drawings.
[0023] FIG. 1 is a graph of Pcm versus weld metal ultimate tensile
strength for a range of compositions of the SBD-AFIM weld metal
according to one embodiment of the present disclosure and that of
the AFIM weld metal disclosed in U.S. Pat. No. 6,565,678.
[0024] FIG. 2 is a cross sectional drawing of a CRC bevel.
[0025] FIG. 3 is a cross sectional drawing of a high strain weld
according to one embodiment of the present disclosure.
[0026] FIG. 4 is a flowchart of a method of welding ferritic steel
pipelines according to one embodiment of the present
disclosure.
[0027] FIG. 5 is a plot of an embodiment of a GMAW pulse waveform
useful in applying an embodiment of the SBD AFIM weld metals.
[0028] FIG. 6 is an optical macro image of a cross-section of an
embodiment of a SBD-AFIM weld illustrating weld fusion defects.
[0029] FIG. 7 is a drawing of a SENT specimen used to generate data
for an R-curve.
[0030] FIG. 8 is a graph of an example R-curve.
[0031] FIG. 9 is a graph of hypothetical R-curves for a low
toughness X70 girth weld and two example high toughness HSWs
according to embodiments of the present disclosure.
[0032] FIG. 10 is a schematic drawing of the SBD-AFIM
microstructure of the weld metal of an embodiment of the present
disclosure.
[0033] FIG. 11 is an optical macro image of an example HSW.
[0034] FIG. 12 is an optical micrograph of the microstructure of
one embodiment of HSW showing SBD-AFIM.
[0035] FIG. 13 is a scanning electron micrograph showing one
embodiment of SBD-AFIM microstructure.
[0036] FIG. 14 is a transmission electron micrograph showing
acicular ferrite and several inclusions, a common component of the
SBD-AFIM microstructure.
[0037] FIGS. 15 and 16 are transmission electron micrographs of
degenerate upper bainite showing several parallel laths and
discontinuous MA at lath boundaries. DUB is a common component of
the SBD-AFIM microstructure.
[0038] FIG. 17 is a transmission electron micrograph of granular
bainite showing multiple grains of bainitic ferrite and scattered
MA particles.
[0039] FIG. 18 is a transmission electron micrograph of lath
martensite showing parallel dislocated laths and no MA at the lath
boundaries.
[0040] FIG. 19 is a graph of Charpy V-notch (CVN) data showing the
effect of CO.sub.2 content in the shielding gas.
[0041] FIG. 20 is a photo of a full-scale pipe strain test failure
location showing pipe collapse away from the girth weld.
[0042] It should be noted that the figures are merely examples of
several embodiments of the present invention and no limitations on
the scope of the present invention are intended thereby. Further,
the figures are generally not drawn to scale, but are drafted for
purposes of convenience and clarity in illustrating various aspects
of certain embodiments of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0043] In the following detailed description section, the specific
embodiments of the present invention are described in connection
with preferred embodiments. However, to the extent that the
following description is specific to a particular embodiment or a
particular use of the present invention, this is intended to be for
exemplary purposes only and simply provides a description of the
exemplary embodiments. The invention is not limited to the specific
embodiments described below, but rather, it includes all
alternatives, modifications, and equivalents falling within the
spirit and scope of the appended claims.
[0044] The weld metal of the present disclosure may be referred to
as strain-based design, acicular ferrite interspersed in martensite
weld metal or SBD-AFIM. Also, when referring to welds containing
this microstructure, the term high strain welds (HSWs) is sometimes
used.
[0045] An embodiment of the present disclosure comprises a ferritic
weld metal that is applied using a modern gas metal arc welding
(GMAW) process with power source current waveform control
sufficient to adequately produce a smooth, controlled welding arc
and weld pool when low quantities of CO.sub.2 (<5%) and oxygen
(<2%) are used in the shielding gas. This produces a ferritic
microstructure useful for SBD pipeline girth welds that are capable
of simultaneously achieving high strength, good low temperature
toughness, excellent ductile tearing resistance and welds with low
defect rates. Embodiments of the present disclosure obtain good
weldability which refers to a group of attributes including good
weld pool fluidity, arc stability ("smooth" arc), good wetting of
the weld pool at the junction with the base metal, and good bead
penetration geometry, all of which are aimed at reducing weld
defects.
[0046] Embodiments of the weld metal discussed in the present
disclosure produce adequate strength and toughness for girth welds
in strain-based design pipelines. These novel welds are suitable
for SBD pipelines in a variety pipe grades, such as, but not
limited to, X52, X60, X65, X70, X80, X90, X100 and potentially X120
and these welds can be applied during field construction with
acceptable weldability and defect rates. The weld metal desired for
a particular application is designed through choice of the weld
metal chemistry and the welding method (process and procedure,
including power source type and shielding gas selection) and can be
applied in conditions of rugged field pipeline construction to
produce suitable weld microstructure and mechanical properties.
[0047] In one embodiment, a weld metal comprises: between 0.03 and
0.08 wt % carbon, between 2.0 and 3.5 wt % nickel, not greater than
about 2.0 wt % manganese, not greater than about 0.80 wt %
molybdenum, not greater than about 0.70 wt % silicon, not greater
than about 0.03 wt % aluminum, not greater than 0.02 wt % titanium,
not greater than 0.04 wt % zirconium, between 100 and 225 ppm
oxygen, not greater than about 100 ppm nitrogen, not greater than
about 100 ppm sulfur, not greater than about 100 ppm phosphorus,
and the balance is iron.
[0048] While the balance of the weld metal composition is iron, it
is possible the weld metal may include other unlisted components,
for example impurities or the like.
[0049] Other elements may be added for the reasons discussed in
further detail below: not greater than about 0.6 wt % copper, not
greater than about 0.04 wt % vanadium, and not greater than 0.6 wt
% chromium, not greater than about 0.04 wt % Nb, not greater than
about 20 ppm B. All percentages herein relating to composition of
the weld metal are expressed in wt % (weight percent).
[0050] Carbon is added to the chemistry as the primary strength
controlling element. Mn contributes solid solution strengthening
and general hardenability, but also acts as a deoxidizer. Ni is
added for its positive influence on toughness. It also contributes
to solid solution strengthening and hardenability. Mo, Cu, and Cr
can be added to boost strength in the solid solution and through
hardenability. Si is added as a deoxidizer and to improve weld pool
fluidity, which helps prevent weld defects. However, Si also
degrades toughness through the formation of oxide inclusions.
Therefore, depending on the tradeoff between toughness and
weldability, Si can be optimized by the user.
[0051] Ti and Zr combine primarily with oxygen in the molten weld
pool, forming small oxides that pin prior austenite grain
boundaries and reduce grain size during cooling from high welding
temperatures. Ti and Zr have a high affinity for oxygen and combine
with oxygen at high temperatures promoting the formation of very
small inclusion nuclei. This promotes the formation of small,
finely dispersed oxides in the weld metal.
[0052] Oxygen is controlled to a great degree by the welding
shielding gas composition (weldability enabled by special power
sources as explained below) when the HSWs are applied with a gas
shielded process. For example, it would be typical to weld the HSWs
with a shielding gas mixture comprised of Ar, He, and 1 to 4%
CO.sub.2 (or 0.5 to 2% O.sub.2). The weld metal oxygen content of
embodiment of the present disclosure balance (1) the need to reduce
non-metallic inclusions in the weld metal to maximize tearing
resistance, and (2) producing a sufficient distribution of
inclusions for the nucleation of acicular ferrite (AF). Efforts to
consistently control oxygen also include cleaning of the weld bevel
(no rust or oily contaminants) and keeping the welding wire stored
and covered to prevent moisture or rust deposits on the wire. In
general, the HSWs are applied using a welding process that controls
oxygen in the welding environment in order to produce optimized and
consistent oxygen levels in the weld pool.
[0053] V and Nb can be added for precipitation strengthening
additions. They combine with carbon and/or nitrogen to form small
carbides, nitrides, or carbonitrides in the weld as a result of
multipass welding. V and Nb can also contribute a small amount to
hardenability and strength. Boron is a powerful strengthening
agent. It can be added to boost strength through interstitial
strengthening and hardenability.
[0054] Sulfur and phosphorus are impurities and are not
intentionally added. Efforts are made to limit these elements in
the weld. Sulfur and phosphorus can be controlled by limiting their
amount in the weld consumable wire. The limits listed above for the
weld metal are also suitable limits for the welding wire.
[0055] Nitrogen is also present as an impurity and typically is
present in the weld metal as a result of atmospheric absorption
during the welding process due to insufficient shielding coverage.
Nitrogen can also be transferred from the weld wire or base metal
dilution. Nitrogen can cause porosity or degraded toughness and its
amount must be limited. The limits listed above for the weld metal
are also suitable for the welding wire.
[0056] Depending on application and the required weld strength, the
weld metal composition can be adjusted within the noted ranges to
suit pipeline grades from X52 to X120. A wide variety of base metal
tensile strengths can be accommodated from about 60 ksi to about
130 ksi. The carbon content is most influential for adjusting
strength, although other alloys can provide some strength
adjustments as well. Lower strengths are achieved with carbon
contents of about 0.03 wt % while the highest strengths are
obtained with carbon contents of about 0.08 wt %. By adjustment of
carbon and other alloys, tensile strengths up to about 150 ksi are
possible. FIG. 1 shows a graph of Pcm versus weld tensile strength
(UTS) for a range of compositions of the novel weld metal. The same
trend for U.S. Pat. No. 6,565,678 is also included in this figure
for comparison. Pcm is a hardenability measure that can be used to
predict strength and the user can adjust chemistry according to
this Pcm data to select a HSW for a particular application. As is
known to those skilled in the art of welding engineering, Pcm can
be calculated based on a known chemical composition.
[0057] High toughness is achievable by the HSWs, even for the
highest strengths versions of embodiments of the present
disclosure. Upper shelf Charpy energy and good CTOD (crack tip
opening displacement) toughness can be achieved down to about
-40.degree. C.
[0058] Due to the low solubility of oxygen in steel welds,
non-metallic inclusions are an important aspect of the
metallurgical design. Whereas conventional pipeline welds are
typically produced with large populations of weld metal inclusions,
the HSWs are designed to control and optimize the type, size, and
density of inclusions. In general, excessive weld metal inclusions
degrade both the brittle and ductile fracture toughness of the HSWs
microstructures provided. These inclusions act as preferential
nucleation locations for both brittle and ductile fracture.
Specifically, for ductile fracture, they provide microvoid
nucleation sites and reduce the energy needed for ductile tearing.
However, in the SBD-AFIM microstructure, the inclusion volume
fraction and size distribution is optimized to achieve high tearing
resistance.
[0059] The microstructure of the SBD-AFIM weld deposit is similar
to that described in U.S. Pat. No. 6,565,678 (the '678 Patent), but
important differences exist. For the purposes of optimizing a weld
for SBD, it has been discovered that while the AFIM weld metal of
the '678 Patent provides a good starting point, this weld metal is
populated with more weld metal inclusions than are required to
nucleate the required volume fraction of acicular ferrite. The
inventors have therefore designed SBD-AFIM weld metals with lower
inclusion content to increase ductile tearing resistance. This has
been accomplished by using shielding gases with lower CO.sub.2
content. Whereas the AFIM weld metals of the '678 Patent would
typically be produced with 5%, 10%, or 15% CO.sub.2, the SBD-AFIM
welds are produced with <5% CO.sub.2. This produces lower oxygen
content and fewer inclusions. The decision to use less than 5%
CO.sub.2 in the shielding gas of SBD-AFIM welds is a key inventive
step. This helps generate a weld with both high brittle and ductile
fracture resistance.
[0060] The use of <5% CO.sub.2 in the HSWs has drawbacks that
the inventors have mitigated. The lower inclusion content that
comes with <5% CO.sub.2 decreases the potential to nucleate
acicular ferrite. Because acicular ferrite is very important for
the SBD-AFIM microstructure, the preferred carbon and total alloy
content is reduced compared to the AFIM welds of the '678 Patent.
This increases the driving force for acicular ferrite which can
offset the lower potential to nucleate acicular ferrite off of
fewer inclusions. A minimum amount of acicular ferrite is necessary
in the SBD-AFIM weld metals to achieve adequate toughness. It is
desirable to produce at least 15% acicular ferrite in the SBD-AFIM
welds. Since ductile tearing resistance is desired for the SBD-AFIM
welds, ideally the welds should contain 20 to 30% acicular ferrite.
Admittedly, the SBD-AFIM microstructure, due to the reduced alloy
content, has less strength making potential compared to the AFIM
microstructure of the '678 Patent. This tendency for lower strength
is mostly with regard to yield strength potential rather than
ultimate tensile strength and the SBD-AFIM microstructure can still
be used for SBD applications where overmatching tensile strength is
a primary goal.
[0061] The SBD-AFIM weld metal chemistry can, in combination with
the base metal chemistry, be used to calculate the necessary
consumable weld wire composition. The SBD-AFIM chemistry can be
applied to a wide variety of base metals simply by alteration of
the weld wire chemistry and knowledge of the welding process that
controls the amount of penetration and base metal dilution. As is
known to those skilled in the art of welding engineering, dilution
calculations can be used to determine one of three chemistries when
two of the chemistries are known or specified. In the case of
welding structural steels, there are three metals involved; the
base metal, the weld metal, and the filler wire. For the
application of 5G mechanized pipeline girth welding, dilution is
typically 10% to 20% for the majority of the weld passes. Dilution
calculations are known in the art and are explained in a number of
welding engineering textbooks including Welding Metallurgy, Volume
2, Third Edition, by George E. Linnert that was published by The
American Welding Society.
[0062] The two primary steps to producing the SBD-AFIM welds
according to embodiments of the present disclosure are (1)
optimizing weld metal oxygen content and (2) limiting weld defects
that might result from welding with lower levels of CO.sub.2 or
O.sub.2 in the shielding gas. Controlling oxygen content is an
important objective because, as described above, the weld metal
needs non-metallic inclusion to nucleate acicular ferrite, but
excessive inclusions lead to degraded ductile tearing resistance.
Optimizing oxygen content is accomplished by limiting the oxygen
potential of the shielding gas (CO.sub.2 or O.sub.2); however, this
choice has a downside. Lowering the oxygen potential, if otherwise
not addressed, results in poor weldability. In particular, <5%
CO.sub.2 in a shielding gas used for mechanized 5G pipeline welding
would typically result in poor weld pool fluidity, arc stability,
bead geometry including penetration profile, all of which can
result in weld defects. This condition is responsible for the
second inventive step for SBD-AFIM welds; limiting defects in light
of the necessary shielding gases.
[0063] Because shielding gases with low oxygen potential are chosen
for SBD-AFIM welds, the weld metal is more viscous when molten and
does not flow or wet as well as typical pipeline weld metals. The
poor weldability makes it difficult to produce smooth transitions
between the weld edges and the base metal. This is often associated
with high surface tension (high viscosity) whereby the junction
between the weld metal and base metal is characterized by a sharp
angle sometimes referred to as a reentrant angle. These regions
(also called the weld toes) can be the location of lack of fusion
defects or they can be trapping sites for silicates that have
floated to the top of the weld pool. This situation can also be
characterized by welds that are "crowned" which refer to a highly
convex weld bead profile.
[0064] In addition to the fluidity problem, the welding arc can be
less stable with reduced CO.sub.2 in the shielding gas. The arc can
sputter and wander to a greater degree and is generally cooler than
an arc with higher CO.sub.2. These aspects also increase the
likelihood of weld defects.
[0065] A typical welding solution used to improve the
aforementioned weldability challenges would be to use welding
shielding gases containing more CO.sub.2 or oxygen. These gases
reduce the surface tension of the weld metal and smooth out the
molten weld pool. These gases also produce better arc stability
which has the effect of creating a smoother weld pool and better
weldability. For the HSWs, using more CO.sub.2 or oxygen is not an
option because this increases inclusions and decreases toughness
and ductile tearing resistance.
[0066] One method for applying the HSWs is using low CO.sub.2 or
oxygen in the shielding gas and this generally means using a higher
amount of argon. Welds made with high levels of argon tend to have
a narrower "finger" penetration bead profile and this increases the
likelihood of weld defects. Helium can be substituted for some of
the argon to reduce the finger penetration bead profile, but helium
also tends to lead to more arc instability which increases the
potential for defects. Therefore, another weldability challenge for
the HSWs is that of preventing excessive finger penetration.
[0067] The two inventive steps key to applying the HSWs can be
accomplished with recently developed welding technology. One
embodiment of the present disclosure utilizes recent advancements
in the electronic control of gas metal arc welding (GMAW) machines
to enable effective application of the HSWs2. The GMAW process is a
typical choice for field pipeline welding because it is rugged and
efficient; however, traditional GMAW equipment requires the
shielding gas contain a significant quantity of either CO.sub.2 or
oxygen to achieve good weldability, i.e., good weld pool fluidity,
arc stability, bead geometry, and low defect rates.
[0068] GMAW welding machines have recently become available that
enable smooth welding (good weldability) of the HSWs with limited
amounts of CO.sub.2 or oxygen in the shielding gas. Using
sophisticated solid state electronics, some manufacturers of GMAW
power sources have recently incorporated advanced pulsed waveform
control to optimize and improve weldability. This type of welding
is generically referred to as pulsed GMAW or PGMAW. The American
Welding Society has designated this process as GMAW-P. Although
PGMAW machines have been in existence for many years, only recently
have waveform controls in these machines become advanced enough to
enable HSWs with the SBD-AFIM microstructure. The inventors have
determined that the newer pulsed waveform welding machines, and
particularly those manufactured after about 2003, enable low oxygen
content and reduced defect potential in spite of the difficulties
that would normally accompany a low oxygen potential shielding
gas.
[0069] For mechanized pipeline girth welding in which the welding
head orbits around the circumference of the pipes being joined, the
HSWs can be deposited in a narrow groove bevel preparation, a weld
design known to those skilled in the art of structural or pipeline
welding. Narrow bevels may be of a single or compound bevel design
whereby the primary bevel is typically of an included angle from
about 0.degree. to about 20.degree.. One common pipeline bevel
design is shown in FIG. 2, which is sometimes called a CRC bevel, a
design pioneered by CRC Evans Automatic Welding, which illustrates
the included angle and the primary bevel surfaces.
[0070] The novel HSW microstructure can also be deposited in an
"open" weld bevel as known to those skilled in the art of
structural or pipeline welding. Open bevels can have included
angles from about 20.degree. up to about 60.degree.. Open bevels
are often used for tie-in welds, repair welds, and insertion of
replacement pipe sections. The HSW microstructure can also be
deposited as a fillet weld or any other weld configuration
depending on the application.
[0071] FIG. 3 is a schematic cross section of an embodiment of the
HSW produced using seven passes. Depending on the application, HSW
technology can be used for all weld passes or for only some weld
passes; if the resultant weld achieves a desired high strain
capacity, it can be termed a HSW. For example, mechanized pipeline
welds are sometimes made where the root pass (pass #1 in FIG. 3) is
deposited from the inside of the pipe using an internal welding
machine. This internal weld bead is typically very small. In one
embodiment of a HSW, the internal root pass can be applied using a
conventional welding wire and procedure while the remainder of the
passes are applied using the SBD-AFIM consumable wire and
chemistry. It can be advantageous to apply the first two passes
(root and hot pass) using conventional technology to reduce the
risk of root defects, and then apply the remaining passes with the
HSWs to produce the SBD-AFIM chemistry. An advantage of a HSW is
the combination of strength and toughness properties, so depending
on the specific structural application and constraints regarding
economic construction scenarios, HSWs can be applied in a variety
of ways to suit the intended purpose.
Welding Process and Procedure Using GMAW
[0072] One embodiment of the present disclosure comprises a method
of producing HSWs for given design conditions. With reference to
FIG. 4, the method comprises determining the desired HSW weld metal
chemistry 61 within the effective ranges disclosed herein. The
method also includes the step of determining the welding consumable
wire chemistry given the base metal chemistry and the desired weld
metal chemistry 62. This step may comprise performing dilution
calculations as discussed previously. The method further comprises
welding the base metal using the welding consumable wire 63,
including the step of providing means for controlling the weld pool
oxygen and inclusion content during welding to achieve a target
weld metal oxygen content and inclusion content 64 and the step of
controlling the arc stability and weld pool flow characteristics
during welding to provide satisfactory weldability and weld fusion
65. The step of controlling the weld pool oxygen content may
comprise cleaning or shielding the weld from elemental oxygen as
well as other oxygen-containing compounds and may include providing
a low-oxygen welding shielding gas or flux. Low oxygen shielding
gas means less than 5% CO.sub.2 and less than 2% oxygen depending
if CO.sub.2 or oxygen is contained in the shielding gas. Low oxygen
flux can be defined, as explained below, through a basicity index
as is known to those skilled in the art of welding engineering. The
step of controlling the arc stability, weld pool flow
characteristics, and bead geometry may comprise use of a modern
pulsed power supply GMAW welding machine with current waveform
control adjusted to permit acceptable weldability of the HSW. This
step may include other welding apparatus and techniques such as
provided below.
[0073] For field pipeline construction, the HSWs are preferably
made using the GMAW-based processes, and particularly PGMAW,
although other processes can be used provided that the specified
chemistry and microstructure are achieved and the weldability and
defect potential (sizes and rate) are satisfactory for the
application. Due to the sensitivity of the HSWs to weld metal
oxygen content and non-metallic inclusions, a preferred welding
technique for achieving the highest levels of toughness with HSWs
is to use a shielding gas composition consisting of mixtures of
argon (Ar), helium (He), and carbon dioxide (CO.sub.2) or oxygen
(O.sub.2). Typical gas compositions range between 7 and 35% He, 1
and 4% CO.sub.2 (or 0.5 and 2% O.sub.2) with the balance Ar. Higher
percentages of He are useful for out-of-position welding and
improved wetting and good bead penetration profile. This must be
balanced with the tendency of He (being a light gas) to be easily
swept away by wind currents during outdoor welding. This can be
managed by using protective welding enclosures if necessary.
Additionally, additions of He can increase variability in the arc
voltage which can lead to arc instability; however, this can be
mitigated, as provided herein, by adjustment of the power supply
and coordination with the characteristics of the welding head.
[0074] Advanced pulsed welding power supplies are important for
achieving the HSW microstructure and achieving good weldability
during field construction. Several examples of these power supplies
are the Fronius TransPulse Synergic 5000, the Lincoln Power Wave
455, and the Miller PipePro 450.
[0075] A system for applying the HSWs to 5G girth welds in an
embodiment of the present disclosure includes the use of background
currents of about 100 to 175 amps and pulse current magnitudes of
about 475 to about 575 amps. Arc voltage typically ranges from
about 16V to about 25V. Wire feed speeds range from about 275 ipm
to about 575 ipm for 0.9 mm diameter wire. Shielding gas flow rates
range from about 30 to about 80 cfh. Travel speeds range from about
25 ipm to about 50 ipm for root and hot pass welding. Travel speeds
range from about 10 ipm to about 25 ipm for the fill passes and
about 8 ipm to about 15 ipm for the cap pass. Filler wire diameters
can range from 0.8 mm to about 1.4 mm. Heat inputs range from about
0.2 kJ/mm to about 0.5 kJ/mm for the root and hot pass and from
about 0.4 kJ/mm to about 1.4 kJ/mm for the fill and cap passes. One
skilled in the art of PGMAW can adjust the pulsing parameters to
obtain the desired welding arc and weld pool that will suppress the
weldability issues associated with low oxygen potential shielding
gas. This adjustment can be accomplished without resorting to the
addition of excessive CO.sub.2 or oxygen to the shielding gas as is
typically practiced for pipeline girth welding.
[0076] As with all situations of weld procedure development when a
new or challenging wire is involved, some experimentation is
necessary to optimize weldability and to limit defect rate. Because
many permutations are possible in combining welding variables, and
because each welding scenario will involve different conditions of
base metal thickness, bevel geometry, and weld position, it is not
practical to prescribe one set of welding parameters that will be
suitable for all applications of HSWs. Routine improvements in
weldability can be made by manipulation of the wire feed speed,
travel speed, shielding gas composition, torch oscillation, and
general arc parameters like background current. Additional
improvements are enabled with modern power sources by adjustment of
the pulsing parameters. This includes, but is not limited to,
adjustment of the following variables: pulse frequency, pulse
magnitude, pulse width, and pulse shape. Due to the rapid response
time of the modern electronics used for waveform control power
supplies, fine adjustments of the pulsing shape can be made
including the shape of pulse ramp up (current rise), peak pulse
current, pulsing current time, overshoot, the shape during ramp
down, tail-out speed, droplet detachment time, step-off current,
droplet detachment current, short circuit current rise, and the
pulse period (frequency). Producing variations such as combining a
series of different pulses is also possible. Additionally,
combining these power source adjustments with the electronics,
motion, or other characteristics of the welding head are also
possible.
[0077] The product literature that accompanies modern waveform
control power supplies contains guidance on how to make pulsing
adjustments to enable specific arc characteristics and weld pool
control. Pulsing adjustments can be used to modify the transfer
mode, the droplet size, the droplet frequency, and to modify such
factors as the turbulence of the weld pool, the weld contour, weld
penetration, and the ability of the weld pool to wet smoothly into
the base metal. In other words, pulsing adjustments can be used to
improve weldability. The pulsing adjustments can also be used to
reduce weld spatter. It is an expected and natural step during weld
procedure development to adjust these parameters to improve
weldability. FIG. 5 illustrates a pulse waveform generated by the
inventors that is useful in applying an embodiment of the SBD-AFIM
weld metals.
[0078] Obtaining the best combination of mechanical properties for
any given HSW geometry and wire combination can be optimized by
adjusting the amount of oxygen in the weld deposit. The inventors
have determined that very low CO.sub.2 (<1%) in the shielding
gas will result in higher strength welds with poor brittle fracture
resistance. Optimal levels of CO.sub.2 (typically 1-4%) produce
welds with high strength and good toughness (both brittle and
ductile fracture resistance). Welds made with higher levels of
CO.sub.2 (>4%) have lower strength and lower ductile fracture
resistance compared to the preferred SBD-AFIM welds.
Weld Pool Agitation
[0079] Weld pool agitation is another technique that can be used to
mitigate or control the weld pool flow characteristics and bead
penetration profile of the HSWs. Mechanical or ultrasonic vibration
can be applied directly to the consumable wire or through an
independent ceramic rod that contacts the molten weld pool. Weld
pool agitation has a similar effect of reducing the surface tension
of the weld pool which enables better weldability. Depending on the
user's capabilities, welding equipment, and fabrication scenario,
the agitation technique can be applied either in addition to, or in
lieu of, using an advance waveform power supply.
Weld Defects
[0080] The HSWs are enabled by use of low oxygen potential
shielding gases and the pitfalls of these shielding gases are
mitigated through use of modern power supplies. These inventive
steps enable mechanized 5G pipeline welds, and even semi-automatic
pipeline welds, to be made with good weldability including good
weld pool fluidity, bead geometry, arc stability and acceptable
defect rates. If the HSWs are attempted without due attention to
optimizing shielding gas and power source control as described
herein, then the welding defect shown in FIG. 6 can occur in sizes
or rates that are unacceptable for efficient pipeline construction.
Typically, it is desirable to keep reject rates due to these
defects below about 5% during pipeline construction. When the HSW
technology is applied properly, then it is possible to maintain
reject rates below 5%. Less than 5% reject rate is considered a low
defect rate.
[0081] With respect to the defect shown in FIG. 6, When the HSWs
are applied properly with due attention to shielding gas and power
source control (including communications with the torch head), then
the sizes of the defects can be limited. Flaw height is a
particularly important dimension of the defects shown. Height is
measured in a direction predominantly perpendicular to the pipe
wall surface. HSWs can be applied while maintaining defect height
to less than 3 mm, or preferably less than 2 mm, even more
preferably less than 1 mm. When the HSWs are optimized to their
maximum potential, defect height can be reduced to less than 0.5 mm
or evenly eliminated completely.
Hybrid Laser Arc Welding
[0082] The HSWs can be applied using the hybrid laser arc welding
(HLAW) process. HLAW welds have high dilution in the lower portions
of the weld metal near the root. In this region, the weld metal is
mostly remelted base metal. Also, this region of the weld
experiences a fast cooling rate. As explained above, dilution
calculations can be used to formulate a suitable HSW filler wire
for any application, and this includes HLAW of structural steels.
Suitable filler wires can be formulated to produce the preferred
weld metal chemistry. Low carbon composition weld wires (not
greater than about 0.05%, more preferably not greater than 0.03%,
and even more preferably not greater than 0.02%) are particularly
useful in creating an appropriate metallurgy for HLAWs that
achieves excellent combinations of strength and toughness.
Submerged Arc Welding
[0083] It is possible to deploy the HSW metallurgy using the
submerged arc welding (SAW) process. One useful application in
pipeline construction is that of double-joining pipes in advance of
the final laying operation. While it is possible to perform
double-joining using the PGMAW techniques mentioned previously, it
is more common to use SAW. To accomplish the desired metallurgy
with the SAW process, special fluxes are required to optimize the
oxygen content of the weld. When SAW welding the HSW metallurgy,
oxygen contents must be kept between about 100 and 225 ppm to
achieve the SBD-AFIM microstructure. This can be done by
controlling the basicity index (BI) of the flux, a term known to
those skilled in the art or welding engineering, an index that
reflects the basic vs. acidic qualities of the flux and its oxygen
removing potential. A number of BI formulas are available, such as
the well-known Tuliani formula.
[0084] Because the application of double-joining is conducted in
the 1G (flat) welding position, this application does not have the
weld metal viscosity problems of out-of-position welding.
Therefore, the need for advanced power supplies is not as great as
for girth welding in the 5G position. It is of course possible to
apply the HSW metallurgy to double joint welds using the previously
mentioned gas metal arc process; however, SAW has productivity
advantages. There exists a tradeoff between the limited position
capabilities of SAW and the weld deposition rate. The deposition
rate can be relatively high, but out-of-position welding is not
possible.
[0085] Strain Based Design Engineering Critical Assessment (SBECA)
for High Strain Welds
[0086] Failure by ductile tearing in SBD applications is a
relatively new design scenario for the pipeline industry, and girth
welds have not previously been engineered to produce high levels of
tearing resistance. The Strain-Based Design Engineering Critical
Assessment (SBECA) technology discussed in this application above
reinforces the importance of weld toughness for SBD pipelines where
higher levels of ductile tearing resistance are useful. This topic
is discussed in the following reference: D. P. Fairchild, et al,
"Girth Welds for Strain-Based Design Pipelines", ISOPE Symposium on
Strain Based Design, the 18th International Offshore and Polar Eng.
Conf, (ISOPE-2008), Vancouver, Canada, Jul. 6-11, 2008, pp.
48-56.
[0087] To optimize HSWs for a particular application, a means of
designing or selecting the appropriate weld properties is
desirable. For SBD pipelines, the following references describe
technology on which SBECA can be based and that can be used to
relate tolerable weld defect size to such factors as applied loads
and material properties: International Patent Application
PCT/US2008/001753; K. Minnaar, et al, "Predictive FEA Modeling of
Pressurized Full-Scale Tests", Proceedings of 17th International
Offshore and Polar Engineering Conference, Lisbon, Portugal, 2007,
pp. 3114-3120; S. Kibey, et al, "Development of a Physics-Based
Approach for the Prediction of Strain Capacity of Welded
Pipelines", Proceedings of 19th International Offshore and Polar
Engineering Conference, Osaka, Japan, 2009; Kibey, S., et al,
"Tensile Strain Capacity Equations for Strain-Based Design of
Welded Pipelines", Proceedings of the 8th International Pipeline
Conference, Calgary, Canada (2010), Fairchild, D. P, et al, "A
Multi-Tiered Procedure for Engineering Critical Assessment of
Strain-Based Design Pipelines", Proceedings of 21st International
Offshore and Polar Engineering Conference, Maui, Hi., 2011. These
references explain how the critical defect size in a weld (the
largest defect that can be tolerated safely) can be calculated
using SBECA technology based on input parameters such as applied
loads or strain, the strength properties of the base metal and
weldment, the toughness properties of the material in which the
defect resides (typically the weld metal or heat affected zone),
and the structural geometry. Alternatively, SBECA technology can be
used to predict the toughness required to support a weld defect of
a given size, given other input parameters such as applied loads,
strength properties, and geometric details.
[0088] For SBD engineering, several candidate methods exist to
measure material toughness including the Charpy V-notch test, the
crack tip opening displacement (CTOD) test, the J-Integral method,
and the curved wide plate test. Research has shown that it is
difficult and/or costly to use these methods to provide a reliable,
predictive parameter relating defect size, applied loads, and
toughness for predictions of structural performance in SBD
scenarios. On the contrary, the SBECA technology above is capable
of quantifying and predicting structural performance, and does so
by using a toughness parameter called the R-curve. This toughness
parameter is measured using a single edge notch tension (SENT) test
as is known by those skilled in the art of mechanics of materials.
References on R-curve testing include: G. W. Shen, et al,
"Measurement of J-R Curves Using Single Specimen Technique on
Clamped SE (T) Specimens", Proceedings of 19th International
Offshore and Polar Engineering Conference, Osaka, Japan, pp. 92-99,
2009; W. Cheng, et al, "Test Methods for Characterization of Strain
Capacity--Comparison of R-curves from SENT/CWP/FS Tests",
Proceedings of 5th Pipeline Technology Conference, Ostend, Belgium,
2009; H. Tang, et al, "Development of the SENT Test for
Strain-Based Design of Welded Pipelines", Proceedings of 8th
International Pipeline Conference, Calgary, Canada, 2010.
[0089] FIG. 7 shows a schematic of a SENT specimen that can be used
to measure an R-curve. Other geometries can be used as well. The
SENT test specimen geometry is similar to a routine tensile test
except that a defect (a crack or notch) is placed at mid-span. The
specimen is gripped at gripping areas. The test procedure includes
pulling the specimen in tension while monitoring and measuring the
progression of defect growth until the specimen can no longer
support significant increases in load. One method for generating an
R-curve involves repeated loading and unloading of the specimen,
where each successive loading cycle imposes increasing loads and
(eventually) increasing crack extension. The progression of crack
extension can be calculated from the compliance of the specimen, a
technique consistent with that described in ASTM E1820 (as
described in the 2012 version). This technique is called the
unloading compliance method and it can be used to relate crack
growth to the applied loads; i.e., the driving force. Any suitable
method of crack growth monitoring can be used such as unloading
compliance or the potential drop method. The data collected can be
used to plot an R-curve graph, which provides a graphical
representation of the toughness, or more specifically, the
materials resistance to ductile tearing. In other words, the graph
characterizes the material's ductile fracture toughness.
[0090] While the SBECA technology referred to herein uses SENT
testing and R-curves to characterize toughness, other methods can
be used to quantify ductile fracture resistance as long as they
provide a quantified, predictive ability to relate key parameters
such as structural geometry, defect geometry, applied loads and
material properties such as strength and toughness properties. One
method is to conduct a series of full-scale pipe strain capacity
tests, although this approach would be very expensive.
[0091] R-curve graphs show the relation between crack extension
versus crack driving force. An example R-curve is shown in FIG. 8.
As the crack extends, the material's resistance to crack growth
(ductile tearing) generally rises. High toughness materials
generate R-curves with steep slopes in the initial part of the
curve and after the initial rise, the R-curve will continue to
rise. The higher the R-curve (larger Y axis values), the higher the
toughness. R-curves are sometimes called "delta a" (.quadrature.a)
curves, or J-integral versus .quadrature.a curves, or CTOD vs.
.quadrature.a curves where the crack driving force is expressed in
terms of CTOD or J-integral and is plotted on the y-axis. Crack
extension .quadrature.a (mm) is plotted on the x-axis. The curves
can be represented by a mathematical relation such as
y=.delta.x.sup..eta., where .quadrature. (delta) and .quadrature.
(eta) are factors in the power law fit of the CTOD (mm) versus
.quadrature.a (mm) plot. According to this description of R-curves
and ductile fracture resistance, the R-curves for different weld
metals can be compared to judge toughness by considering the CTOD
at a crack extension of 1 mm. There are two reasons to select a
crack extension of 1 mm for such comparisons. First, when x=1 in
the power law equation, the power term reduces to 1 and eta can be
ignored. Then, the CTOD is equal to delta and comparisons can be
made using only the value of delta. Second, 1 mm of crack growth is
a reasonable degree of crack growth to compare toughnesses.
According to SBECA knowledge, the strain capacity of pipe girth
welds often occurs when crack extensions are on the order of 1 mm.
Critical crack extensions can vary from very small values up to 1
mm or 2 mm, depending on many geometry and material property
factors, but for the purposes of making general toughness
comparisons, the 1 mm convention is adequate.
[0092] R-curves of the novel HSW weld metal can generate delta
values of more than 0.75 at tensile strengths as high as 150 ksi.
With good control of oxygen content or for the lower strength
versions of the HSWs, delta values can be greater than 1.00.
Depending on application, attention can be focused on optimal
welding conditions as disclosed herein and delta values of 1.25 can
be achieved or even 1.5 or 1.75. The HSW weld metal can produce
these high toughnesses while simultaneously providing high
strengths suitable for overmatching X52, X60, X65, X70, X80 or
stronger pipe grades for SBD pipelines.
[0093] The ability to accurately predict structural performance
based on R-curve data and SBECA technology depends on validation of
the technique using full-scale pipe strain capacity tests. This is
discussed in the following references: International Patent
Application PCT/US2008/001676; P. Gioielli, et al, "Large-Scale
Testing Methodology to Measure the Influence of Pressure on Tensile
Strain Capacity of a Pipeline, Proceedings of 17th International
Offshore and Polar Engineering Conference, Lisbon, Portugal, 2007,
pp. 3023-3027; P. C. Gioielli, et al, "Characterization of the
Stable Tearing During Strain Capacity Tests", ISOPE Symposium on
Strain Based Design, the 18th International Offshore and Polar Eng.
Conference, (ISOPE-2008), Vancouver, Canada, Jul. 6-11, 2008, pp.
86-89; X. Wang, et al, "Validation of Strain Capacity Prediction
Method--Comparison of Full-Scale Test Results to Predictions from
Tearing Analysis Based on FEA", Proceedings of 5th Pipeline
Technology Conference, Ostend, Belgium, 2009. Validation enables
relating R-curve data to full-scale performance and this connection
provides a calibration basis for parametric development of
predictive mathematical expressions for SBECA.
[0094] The inventors have used the SBECA technology to quantify the
effect of ductile fracture resistance in terms of R-curves for SBD
scenarios involving a variety of pipe grades, defect sizes, weld
properties, and base metal properties, including consideration of
internal pipe pressure and pipe misalignment at the girth welds. A
hypothetical example of the results from this work for an X70 girth
weld is shown in FIG. 9. This example is for a 42 inch, 20 mm wall
pipe with the following longitudinal tensile properties: yield
strength of 75 ksi, ultimate tensile strength of 85.2 ksi, and a
uniform elongation of 8%. The target strain capacity is 2.5%. Three
hypothetical welds are considered, all with 20% UTS (ultimate
tensile strength) overmatch and zero millimeters misalignment. For
these three welds, the graph displays three different R-curves
representing different levels of ductile tearing resistance (all
other properties remaining equal). By considering the R-curve
values at 1 mm of crack extension, the three curves have delta
values of 0.6, 1.3, and 2.0. These levels of tearing resistance
represent a relatively low toughness weld (0.6), and two HSWs
called HSW #1 and HSW #2. Using the disclosed SBECA technology,
critical defects can be calculated for these three R-curves. In
terms of defect depth and length in millimeters, the three critical
defect sizes associated with the three R-curves are 3.3.times.20
mm, 4.3.times.48 mm, and 6.4.times.50 mm, respectively. As can be
seen, higher levels of tearing resistance provide greater defect
tolerance. SBECA technology can be used as a design aid to select
an optimum set of mechanical properties for HSWs.
[0095] HSWs can be designed to produce a range of strengths.
Because strength and toughness are inversely related in structural
steels, creating higher strength generally means producing lower
toughness. For this reason, it is generally not desirable to create
any more weld strength than is needed for the application because
lower toughness is the tradeoff SBECA technology can be used to
design HSWs and optimize the tradeoff between strength and
toughness.
Weld Metal Microstructure
[0096] Definitions of metallurgical terms describing the HSW
microstructures may be found in the Glossary, while additional
details are described in the following three references: (1) N. V.
Bangaru, et al, "Microstructural Aspects of High Strength Pipeline
Girth Welds," Proceedings of the 4.sup.th International Pipeline
Technology Conference, Ostend, Belgium, May 9-13, 2004, pp.
789-808, (2) J. Y. Koo, et al, "Metallurgical Design of Ultra-High
Strength Steels for Gas Pipelines," ISOPE Symposium on
High-Performance Materials in Offshore Industry, the 13th
International Offshore and Polar Eng. Conference, (ISOPE-2003),
Honolulu, Hi., USA, May 25-30, 2003, pp. 10-18, and (3) U.S. Pat.
No. 6,565,678. As used herein, predominant or predominantly means
at least about 50 volume percent.
[0097] In stress-based pipeline design, the microstructure of
choice for girth welds is generally acicular ferrite. Furthermore,
for high strength pipelines of stress-based design, the
microstructure of he '678 Patent is useful. The microstructure of
the weld metal of the present disclosure is different from both of
these examples. The microstructure of the current invention is
comprised of an AFIM microstructure, but the inclusion content is
lower than disclosed by the '678 Patent. Whereas the '678 Patent
teaches that inclusion number densities of about 5.times.10.sup.10
m.sup.-2 to 6.times.10.sup.10 m.sup.-2 are beneficial, the SBD-AFIM
weld metal requires less than 4.times.10.sup.10 m.sup.-2.
[0098] The inventors have studied many variations of AFIM and
SBD-AFIM microstructures in detail, and have discovered that the
best combination of properties for the intended SBD application is
achieved with a balanced proportion of hard constituents and
acicular ferrite. A schematic of the SBD-AFIM microstructure is
shown in FIG. 10. An example SBD-AFIM weld is shown in FIG. 11. An
optical micrograph showing the SBD-AFIM microstructure is shown in
FIG. 12. A scanning electron micrograph of the SBD-AFIM
microstructure is shown in FIG. 13. A transmission electron
micrograph of acicular ferrite is shown in FIG. 14. The hard
constituents in SBD-AFIM are predominantly mixtures of lath
martensite, lower bainite, degenerate upper bainite, and granular
bainite. Transmission electron micrographs of several of these
constituents are shown in FIGS. 15-18.
[0099] During weld cooling, Ti and Zr based inclusions form in the
molten weld metal. These base inclusions are typically further
enveloped by spinel shells. As the weld metal cools further,
acicular ferrite is nucleated on these inclusions. The remaining
austenite then transforms to a mixture of the hard constituents.
Typical microstructural balances for SBD-AFIM welds are 15% to 50%
acicular ferrite and greater than 50% of the hard constituents.
This represents a somewhat higher content of acicular ferrite than
described for the typical AFIM welds of the '678 Patent.
Weld Inspection
[0100] The HSWs described herein have advantages related to weld
inspection as compared to the welds described in U.S. Patent
Application Publication No. US PA 2010/0089463, published Apr. 15,
2010 (International Patent Application PCT/US2008/001409). The HSWs
are ferritic whereas the welds of US PA 2010/0089463 are Ni-based
welding consumables and they produce austenitic welds which have a
face centered cubic (FCC) atomic structure. The ferritic HSWs have
a body centered cubic (BCC) atomic structure which is useful in the
welding of ferritic pipeline steels (which are also BCC in
structure) because it avoids the problem of the dissimilar weld
interface that occurs with using high Ni (FCC) welding consumables
to weld ferritic pipeline steels. Dissimilar weld interfaces cause
difficulties in ultrasonic inspection, as these interfaces produce
false signals which can result in unnecessary repairs.
Examples
[0101] The welding wires listed in Table 1 have been made by the
inventors for experimentation of SBD-AFIM welds.
TABLE-US-00001 TABLE 1 Weld wire chemistries Wire C Mn Ni Mo Cu Cr
Si Ti Zr Pcm 1 0.035 1.81 2.94 0.58 0.12 0.08 0.610 0.014 0.027
0.244 2 0.045 1.86 2.98 0.57 0.30 0.16 0.600 0.014 0.024 0.269 3
0.065 1.9 3.07 0.59 0.16 0.20 0.390 0.014 0.040 0.282
[0102] Using wires 1, 2, and 3, several 1G and 5G girth welds were
produced using the SBD-AFIM technology disclosed herein. These
welds were made on 30 inch diameter, 15.6 mm wall API 5L X80 pipe.
This pipe was of the following composition by weight % (wt. %):
Carbon: 0.06, Mn: 1.88, Si: 0.25, P: 0.006, S: 0.002, Ni: 0.17, Cu:
0.18, Mo: 0.22, Cr: none, Nb: 0.03, V: none, and Ti: 0.01. The
welds were produced using CRC Evans automatic welding equipment
which included use of a Fronius TransPulse Synergic 5000 power
supply. The CO.sub.2 content of the shielding gas was varied from 0
to 3%. The pulsing parameters were adjusted as disclosed herein,
and good weldability was achieved along with excellent mechanical
properties. Typical heat inputs in the fill passes of these welds
ranged from about 0.45 kJ/mm to 0.70 kJ/mm. Additional details
about these welds are given in Tables 2 and 3. The tensile values
provided in Table 2 are an average of either two or three tests.
The Charpy V-notch (CVN) values in Table 2 are extracted from curve
fits to full transition curves whereby each curve was established
using approximately 15 individual CVN tests spread across five test
temperatures (-60 C, -40 C, -20 C, 0 C, 22 C). The CTOD values
given in Table 2 are a minimum of three tests.
TABLE-US-00002 TABLE 2 Welding Details and Mechanical Properties
Weld Yield CVN CTOD Posi- He Ar CO2 Strength UTS (J) @ (mm) @ Weld
Wire tion (%) (%) (%) (ksi) (ksi) -20.degree. C. -20.degree. C. 1 1
5G 30 70 0 119 131 98 0.04 2 1 5G 30 69 1 116 136 143 0.08 3 1 5G
30 68 2 121 133 177 0.19 4 1 5G 30 67 3 111 129 189 0.19 5 1 1G 30
68 2 120 131 162 0.34 6 1 5G 10 87 3 127 136 189 -- 7 2 5G 30 69 1
116 143 124 0.11 8 2 5G 30 68 2 116 143 178 0.15 9 3 5G 30 69 1 121
146 98 0.09 10 3 5G 30 68 2 117 151 156 0.14
TABLE-US-00003 TABLE 3 Weld Metal Chemistries Si Mo Ni Cu O Weld
Wire C (%) Mn (%) (%) Cr (%) (%) (%) (%) Ti (%) Zr (%) (ppm) Pcm 1
1 0.038 1.8 0.52 0.079 0.57 2.56 0.13 0.012 0.025 130 0.236 2 1
0.034 1.78 0.53 0.079 0.57 2.61 0.13 0.011 0.024 180 0.233 3 1
0.040 1.8 0.65 0.078 0.56 2.57 0.13 0.01 0.024 180 0.242 4 1 0.037
1.79 0.67 0.082 0.58 2.72 0.12 0.001 0.026 240 0.243 5 1 0.046 1.85
0.51 0.081 0.55 1.82 0.15 0.011 0.026 140 0.239 6 1 not measured 7
2 0.050 1.9 0.53 0.14 0.55 2.4 0.25 0.012 0.023 110 0.259 8 2 0.049
1.86 0.56 0.15 0.59 2.96 0.27 0.011 0.024 200 0.270 9 3 0.057 1.92
0.48 0.19 0.61 2.9 0.17 0.012 0.037 120 0.276 10 3 0.059 1.9 0.34
0.15 0.45 2.91 0.13 0.008 0.026 160 0.258
[0103] The optimal CO.sub.2 content was found to be approximately
2-3% while lesser CO.sub.2 content showed degraded properties
compared to the 2-3% CO.sub.2 welds. The weldability was also found
to be optimal with 2-3% CO.sub.2 compared to the welds with less
CO.sub.2 content. Welds number 1 through 4 are a good example of
the effect of CO.sub.2 content in the shielding gas. As the
CO.sub.2 content increased in order 0%, 1%, 2%, and 3%, the CVN
value at -20 C changed in order 98J, 143J, 177J, and 189J,
respectively. This trend is shown in FIG. 19 where the
ductile-to-brittle transition temperature decreases as the CO.sub.2
content increased from 0% to 3%. It is noted that the oxygen
content of welds 2 and 3 (with CO.sub.2 contents 1% and 2%,
respectively) are the same at 180 ppm. It would normally be
expected that the lower CO.sub.2 content weld would generate a
lower oxygen content; however, as is known to those skilled in the
art of welding metallurgy, scatter in weld metal oxygen content
measurements is quite common. Nevertheless, the overall trend in
the data shown in Tables 2 and 3 demonstrates a novel aspect of the
SBD-AFIM disclosed herein.
[0104] Welds 1 through 4 show that as CO.sub.2 content increases
from 0% to 3%, CTOD increases. Specifically, as the CO.sub.2
content increased in order 0%, 1%, 2%, and 3%, the CTOD values
changed in order 0.04 mm, 0.08 mm, 0.19 mm, and 0.19 mm,
respectively.
[0105] The weld pair 7 & 8 and the weld pair 9 & 10
demonstrate the aforementioned toughness trend. Namely, as CO.sub.2
content in the gas increased from 1% to 2%, both of these example
pairs (with different weld wires) show that toughness increases.
Both CVN and CTOD toughness increased. The weld metal oxygen
content data for these pairs is consistent with the stated trend
whereby oxygen increases as CO.sub.2 content increases. The trend
in oxygen content requires explanation. Conventional thinking
states that as oxygen content increases, weld metal toughness
decreases. This is generally true, but as the details of the
present disclosure show, and within the relevant range of CO.sub.2
content for SBD-AFIM (less than 5%), there are two competing
factors. Decrease in CO.sub.2 content leads to decreased weld metal
oxygen and generally this would increase toughness, but for the
SBD-AFIM welds there is a competing effect of acicular ferrite
nucleation. When the weld metal oxygen content becomes too low,
acicular ferrite nucleation is stifled and toughness decreases.
Therefore, it is demonstrated that an optimal tradeoff exists
between the competing factors and 2-3% CO.sub.2 content in the
shielding gas is the best balance.
[0106] A full-scale pipe strain test was conducted using 30 inch
diameter, 15.6 mm wall thickness X80 pipe. The specimen contained
two girth welds made using the welding scenario described as weld 3
in Table 2. The full-scale specimen was pre-populated with a total
of four 3.times.50 mm defects; two in each girth weld. The defects
were machined from the OD (outside diameter) and were placed in
both the weld metal and heat affected zone. The girth welds were
produced with up to 3 mm of high-low misalignment and the defects
were placed in the location of maximum misalignment. A companion
girth weld was produced and used for property measurement. The
yield and ultimate strength of the weld metal was 119 ksi and 130
ksi, respectively. The CVN toughness at -20 C was 179J. Three CTOD
tests at -20 C produced values of 0.18 mm, 0.30 mm and 0.25 mm.
Several SENT tests produced an average R-curve delta value of 1.25.
The full-sale test was pressurized to 72% of the specified minimum
yield strength and pulled in tension to failure. This test was
conducted as explained in the previously cited references on
full-scale pipe strain testing. The test achieved 3.2% strain
before the specimen failed in the pipe body away from the weld. A
photo of the failure location is shown in FIG. 20.
[0107] As demonstrated by these examples, HSWs are useful in
producing pipeline girth welds capable of achieving high
toughnesses and high levels of applied strain even when containing
common welding defects. HSWs can be made with tensile strengths as
high as 100 ksi, 110 ksi, 120 ksi, more preferably 130 ksi, and
even more preferably 140 ksi. These welds can produce good brittle
fracture resistance as evidenced by weld metal CTOD values above
0.10 mm at temperatures of -20.degree. C. for welds made using
optimized conditions. With attention paid to chemistry, oxygen
content, and microstructure, HSWs can produce this strength and
toughness at temperatures as low as -10.degree. C., -15.degree. C.,
-20.degree. C., or even -30.degree. C. or -40.degree. C.
[0108] Another useful measure of toughness is the
ductile-to-brittle transition temperature, a common parameter known
to those skilled in the art of welding metallurgy and structural
design. This transition temperature can be determined using any
number of tests including the Charpy V-notch (CVN) test. The CVN
transition temperature can based on either Charpy energy or shear
area and generally refers to the middle or mid-point of the
toughness transition curve on a graph of Charpy toughness versus
test temperature. The transition temperature of HSWs as measured by
the CVN test can be made to produce ductile-to-brittle transition
temperatures down to -20.degree. C., -30.degree. C., or -40.degree.
C. With attention paid to chemistry, oxygen content, and
microstructure, transition temperatures as low as -60.degree. C. or
-80.degree. C. can be achieved. The upper shelf energies produced
by the HSWs can be 100J, more preferably 125J, even more preferably
150J. If the HSW is designed with optimized levels of oxygen and
acicular ferrite content, then upper shelf energies of 175J can be
achieved.
[0109] With respect to ductile fracture resistance, the HSWs can
produce R-curves as high or higher than described by a curve where
at a crack extension of 1 mm the delta value is at least 0.75. With
attention paid to chemistry, oxygen content and microstructure,
HSWs can produce R-curves as high or higher than a curve with a
delta value of 1.0, preferably 1.25, and even higher than 1.5.
[0110] With the above described mechanical properties, the HSW
girth welds can achieve global plastic strains greater than 0.5%
while containing typical weld defects of sizes such as 2.times.25
mm, 3.times.50 mm, 4.times.50 mm, or 5.times.50 mm, or 6.times.50
mm, depending on wall thickness. The first dimension of these
defects describes the flaw height in a direction perpendicular to
the pipe surface and the second dimension (the larger dimension) is
the flaw length along the hoop direction of the girth weld. Even
long defects such as 3.times.100, 2.times.100 mm or 1.times.200 mm
can be supported while achieving plastic strains larger than 0.5%.
Depending on defect size and pipe wall thickness, global plastic
strains of 1%, 1.5%, 2%, 2.5%, 3% or even 4% or 5% can be achieved.
High strain capacities can be achieved in pipe grades up to about
X120.
[0111] It should be understood that the preceding is merely a
detailed description of specific embodiments of this invention and
that numerous changes, modifications, and alternatives to the
disclosed embodiments can be made in accordance with the disclosure
here without departing from the scope of the invention. The
preceding description, therefore, is not meant to limit the scope
of the invention. Rather, the scope of the invention is to be
determined only by the appended claims and their equivalents. It is
also contemplated that structures and features embodied in the
present examples can be altered, rearranged, substituted, deleted,
duplicated, combined, or added to each other. The articles "the",
"a" and "an" are not necessarily limited to mean only one, but
rather are inclusive and open ended so as to include, optionally,
multiple such elements.
Glossary
[0112] Austenitic alloys: any of a group of engineering alloys such
as stainless steel, Ni-based alloys, and duplex stainless steels
that possess an austenitic microstructure characterized by a face
centered cubic (fcc) atomic arrangement.
[0113] Ferritic alloys: any of a group of engineering alloys that
possess a ferritic microstructure characterized by a predominantly
body centered cubic (bcc) atomic arrangement.
[0114] Yield strength: That strength corresponding to a departure
from linear elastic behavior where load support is achieved without
permanent deformation and plastic behavior where load support
results in measurable permanent deformation.
[0115] Tensile strength: That strength corresponding to the maximum
load carrying capability of the material in units of stress when
the failure mechanism is not linear elastic fracture.
[0116] HAZ: Heat-affected-zone.
[0117] Pcm: A formula used to quantify hardenability based on the
wt % of common alloying elements used in steel. Hardenability is
the degree to which a steel transforms to martensite (a hard
microstructure) when cooled from high temperatures.
Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B.
The alloy content in wt. % is entered into the equation to
calculate the Pcm number.
[0118] Heat-affected-zone: Base metal that is adjacent to the weld
fusion line, is not melted during the welding operation, but that
was affected by the heat of welding.
[0119] Toughness: Resistance to fracture.
[0120] Weldment: An assembly of component parts joined by
welding.
[0121] Weld bead penetration profile: The shape of the weld bead
near the bottom (root) of the weld bead when observed in a
transverse cross-section.
[0122] Weldability: The feasibility of welding a particular metal
or alloy. Sometimes weldability refers to the susceptibility of
hydrogen induced cracking during welding, but in the context of
this disclosure, weldability refers to the ease of welding without
creating defects such as lack of fusion, lack of penetration, or
undercut. A number of factors contribute to poor weldability
including a high surface tension molten weld pool and an erratic or
unstable welding arc. These factors create symptoms observed by the
welder including poor wetting of the weld pool into the adjacent
base metal, sharp (or small) reentrant angles at the weld toes,
undesirable weld spatter. Obtaining good weldability refers to a
group of attributes including good weld pool fluidity, arc
stability ("smooth" arc), good wetting of the weld pool at the
junction with the base metal, good bead penetration geometry all of
which are aimed at reducing weld defects.
[0123] Gas metal arc welding (GMAW): A welding process that
utilizes a torch whereby the filler wire acts as the electrode, is
automatically feed through a contact tip, and is consumed in the
welding process. The contact tip is typically surrounded by a gas
cup that directs shielding gas to the area of the welding arc.
Common shielding gases are argon, CO2, helium, and oxygen. Torch
travel can be provided by a machine (automatic or mechanized) or
can be provided by a human (semiautomatic). The process name GMAW
is a standard designation made by the American Welding Society.
[0124] Pulsed gas metal arc welding (PGMAW): A variation of the
GMAW process that utilizes power sources that provide current
pulsing capabilities. These are sometimes referred to as advanced
current waveform power sources. The American Welding Society has
termed PGMAW as GMAW-P.
[0125] GMAW-based processes: A number of allied processes similar
to GMAW such as PGMAW, metal core arc welding (MCAW), and flux
cored arc welding (FCAW). The primary difference with MCAW is that
a cored wire is used and there exists metal powders within the
core. The FCAW process also uses a cored wire and the core
typically consists of flux powders. FCAW can be used with or
without shielding gas.
[0126] Gas tungsten arc welding (GTAW): A welding process that
utilizes a torch whereby the electrode is a non-consumable tungsten
rod. The process can be performed with or without a filler wire. If
without a filler wire, the process is referred to as autogenous. If
a filler wire is used, it is fed from the side (as opposed to
through the torch centerline as with the many other processes like
GMAW) into the weld pool/arc region. Filler wire feed can be
provided by a machine or by a human. Weld torch travel can be
provided by a machine or by a human. The tungsten electrode is
surrounded by a gas cup that directs shielding gas to the weld
pool/arc region. Typical shielding gases include argon and
helium.
[0127] Hybrid-laser arc welding (HLAW): A process that combines
laser welding and GMAW. Typically the laser precedes the GMAW arc
to provide deep penetration. The GMAW component of HLAW creates the
ability to accommodate larger variations in joint fit up as
compared to laser welding alone. Whereas a laser can only bridge
gaps of very narrow widths (.about.1 mm), GMAW welding can bridge
gaps of several millimeters.
[0128] Submerged arc welding (SAW): A welding process that requires
a continuously fed consumable solid or tubular (flux cored)
electrode. The molten weld and the arc zone are protected from
atmospheric contamination by being "submerged" under a blanket of
granular fusible flux.
[0129] Low oxygen welding environment: A welding process whereby
the protection afforded to the molten weld pool achieves a weld
metal oxygen content of less than about 200 ppm oxygen. The
protection can be achieved by use of a shielding gas or a flux.
[0130] Proeutectoid ferrite (PF): In reference to steel weld
microstructures, this phase is also called polygonal ferrite and
grain boundary ferrite. PF tends to be one of the first; if not the
first phase to transform from the austenite as the weld metal cools
from high temperatures. Nucleation occurs at the prior austenite
grain boundaries; therefore the PF grains are located on these
boundaries. The grains can take on a polygonal shape or sometimes
sideplates will form from the allotriomorphs which then defines a
related phase called Widmanstatten ferrite.
[0131] Acicular ferrite (AF): AF is often the first decomposition
product to transform in a steel weld from the austenite during
cooling, although proeutectoid ferrite (polygonal ferrite) can
sometimes form first. AF nucleates on small, non-metallic
inclusions and then experiences rapid growth by a bainitic-type
transformation mechanism. The AF grains typically exhibit a
needle-like morphology with aspect ratios ranging from about 2:1 to
20:1 depending on cooling rate and chemistry. This transformation
involves both shear and diffusional components. The transformation
temperature controls the interplay between the diffusional and
shear components, thus determining AF morphology.
[0132] Granular bainite (GB): Refers to a cluster of 3 to 5
relatively equiaxed bainitic ferrite grains that surround a
centrally located, small "island" of Martensite Austenite (MA).
Typical "grain" diameters are about 1-2 .mu.m.
[0133] Upper bainite (UB): Refers to a mixture of acicular or laths
of bainitic ferrite interspersed with stringers or films of carbide
phase such as cementite. It is most common in steels with carbon
contents higher than about 0.15 wt %.
[0134] Degenerate upper bainite (DUB): A bainitic product where
each colony grows by shear stress into a set (packet) of parallel
laths. During and immediately after lath growth, some carbon is
rejected into the interlath austenite. Due to the relatively low
carbon content, carbon enrichment of the entrapped austenite is not
sufficient to trigger cementite plate nucleation. Such nucleation
does occur in medium and higher carbon steels resulting in the
formation of classical upper bainite (UB). The lower carbon
enrichment at the interlath austenite in DUB results in formation
of martensite or martensite-austenite (MA) mixture or can be
retained as retained austenite (RA). DUB can be confused with
classical upper bainite (UB). UB of the type first identified in
medium carbon steels decades ago consists of two key features; (1)
sets of parallel laths that grow in packets, and (2) cementite
films at the lath boundaries. UB is similar to DUB in that both
contain packets of parallel laths; however, the key difference is
in the interlath material. When the carbon content is about
0.15-0.40, cementite (Fe3C) can form between the laths. These
"films" can be relatively continuous as compared to the
intermittent MA in DUB. For low carbon steels, interlath cementite
does not form; rather the remaining austenite terminates as MA,
martensite, RA, or mixtures thereof.
[0135] Lower bainite (LB): LB has packets of parallel laths similar
to DUB. LB also includes small, intra-lath carbide precipitates.
These plate-like particles consistently precipitate on a single
crystallographic variant that is oriented at approximately
55.degree. from the primary lath growth direction (long dimension
of the lath).
[0136] Lath martensite (LM): LM appears as packets of thin parallel
laths. Lath width is typically less than about 0.5 Untempered
colonies of martensitic laths are characterized as carbide free,
whereas auto-tempered LM displays intra-lath carbide precipitates.
The intralath carbides in autotempered LM form on more than one
crystallographic variant, such as on <110> planes of
martensite. Often the cementite is not aligned along one direction;
rather it precipitates on multiple planes.
[0137] Tempered martensite (TM): TM refers to the heat treated form
of martensite in steels whereby the heat treatment is performed in
furnace or by local means such as using heating wrap. This form of
tempering is conducted after welding fabrication. The
microstructure and mechanical properties change as the metastable
structure martensite incurs the precipitation of cementite during
excursions in a temperature range where cementite precipitation is
possible, but too low for austenite formation.
[0138] Auto-tempered lath martensite: martensite that incurs
self-tempering during cooling from an operation such as welding.
Cementite precipitation occurs in-situ, on cooling, and without
reheating as is done for traditional tempering.
[0139] Twinned Martensite (TwM): This version of martensite forms
due to a higher carbon content compared to chemistries that
contains mostly lath martensite. TwM forms when the carbon content
is above about 0.35% to 0.44%. Below this carbon level, lath
martensite is predominant. TwM contains internal twins that have
formed to accommodate transformation deformations and stresses.
Typical structural steels do not contain high carbon contents;
therefore, TwM in structural steels (particularly welds) is mostly
found in regions of chemical segregation. Segregation can create
local areas of high carbon concentration, thus leading to TwM. This
is often the case in areas of MA in welds and heat affected
zones.
[0140] Martensite austenite constituent (MA): Remnant areas of
microstructure in a ferritic steel or weld that transform on
cooling to a mixture of martensite and retained austenite. These
areas are often the last regions to transform on cooling. MA
regions are stabilized due to carbon rejection from surrounding
areas that have already transformed at higher temperatures. Due to
stabilization, the transformation of austenite to MA occurs at
lower temperatures than the surrounding areas. Regions of MA are
typically dominated by martensite while only containing small
volume fractions of retained austenite (less than 10%). MA is often
seen on prior austenite grain boundaries of welds or HAZs that
experience double thermal cycles. MA is also found on lath
boundaries in the lath based microstructures of degenerate upper
bainite and lower bainite. MA is typically observed on any number
of lath, packet or grain boundaries present in structural
steels.
[0141] Retained Austenite: Austenite that remains in the steel
microstructure after cooling to room temperature. Austenite is
stable at high temperatures, but once the microstructure cools
below the A3 and A1 temperatures, lower temperature transformation
products, such as ferrite, bainite and martensite, become stable
and form from the austenite. Depending on cooling rate and
chemistry, some small areas of the microstructure can become
enriched in alloys (mostly carbon) and they remain stable and
present at room temperature.
[0142] Engineering Critical Assessment (ECA): Methods for
designing, qualifying, or otherwise assessing the structural
significance of material defects, such as cracks or weld defects.
One goal is to prevent structural failure. Another goal is prevent
unnecessary repairs when the defects are analyzed to be benign. ECA
methods are often based on fracture mechanics technology. ECA
methods are capable of defining the critical conditions for failure
based on, generally, three inputs: material properties, applied
loads, and defect size. ECA is often used to predict the critical
value of one parameter based on input of the other two. Other names
for ECA methods include defect assessment procedures and
fitness-for-purpose analysis.
[0143] Strain Based Engineering Critical Assessment (SBECA):
Methods to determine the flaw tolerance of pipeline girth welds to
applied tensile strains. This may mean characterizing ductile
fracture resistance by experiments and then calculating acceptable
flaw sizes based on a target strain demand. Alternatively, a target
strain demand and flaw size can be used to calculate required
ductile fracture resistance. SBECA requires knowledge or
assumptions regarding several material properties including yield
and tensile strengths. Often assumptions are necessary regarding
the accuracy of non-destructive inspection techniques.
[0144] Critical defect size: Reference to a material defect, such
as a crack or weld defect, in an engineering structure where this
defect is the smallest defect that will cause failure depending on
the specifics of pipe and weld mechanical properties, defect
geometry, structural geometry, and applied loads. This term is
commonly used when discussing engineering critical assessment
(ECA).
[0145] High-Low Misalignment: the degree of geometric offset
between adjacent pipe pieces at a girth weld. Misalignment varies
around the pipe circumference. While best efforts are made to
minimize misalignment, the magnitude of high-low can be fractions
of a millimeter up to several millimeters. 1 mm of high-low would
be considered small for large diameter pipe (say, for >24''
diameter pipe), while >3 mm of high-low would be considered
large. High-low misalignment rarely exceeds about 5 mm.
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